1. Technical Field
The present invention relates generally to communication networks, and more particularly to the management of packet transmission errors.
2. Related Art
There is a continuously growing need to exchange information via communication networks in order to transmit larger and larger data files. This phenomenon is accentuated with the development of the multimedia applications. This explains why the variety of communication networks now available has a common objective: rapidity and efficiency of the transmission. Actually, these characteristics are required in order to consume less transmission resource and thus to allow more and more users to communicate through any communication network and to transmit increasing volumes of data. In addition, reliability of the transmission appears to be a key characteristic. On the other hand, the transmission error rate depends on the type of medium used by communication networks. In particular, wireless medium is generally not very reliable. Indeed, these wireless networks are prone to relatively high error levels.
As a result, this type of networks integrates an Error Control (EC) entity to deal with their intrinsic high transmission error levels. In usual networks based on communication protocols stack according to the OSI (“Open System Interconnect”) model of ISO (“International Standardization Organization”), an EC entity is included in the Data Link Layer (LL) in order to manage retransmissions of corrupted packets. In the following description, the term “resource” will be referred to as “transmission resource”.
Classically, an EC entity is in charge of guaranteeing correct packet transmission. Stated otherwise, an EC entity manages the retransmission of corrupted or missing packets in case of transmission errors. Many types of EC entities have heretofore been proposed for many types of networks. However, in the following description, will only be considered wireless networks because they are the most critical networks due to their not really reliable transmission medium. Moreover in such networks, the transmission resource is limited. As a result, the efficiency of an EC entity becomes a key aspect. Regarding the foregoing, an efficient Medium Access Control (MAC) layer is required to share the resource provided by the PHYsical (PHY) layer without adding too much signalling overhead.
The following will consider the types of EC entity already available in the prior art. The direction used to transmit data will be referred to as “Forward direction”, whereas the reverse direction used to return feedback information will be referred to as “Backward direction”. An EC entity based on an Automatic Repeat reQuest (ARQ) protocol is usually used to perform a data transmission providing an error-free service to the upper layer. An ARQ protocol is used in an EC entity for data packets transmission in which the receiver can detect a transmission error and then automatically transmits a repeat request to the transmitter. As a result, the transmitter retransmits the corresponding data packets until they are either correctly received or the number of retransmission attempts exceeds a predetermined threshold.
Generally, the ARQ protocols rely on a packet identification scheme common to the transmitter and the receiver, so that the receiver can indicate to the transmitter, which packets are not correctly received through a feedback information message. The packet identification is typically an incremental Sequence Number (SN) identifier. In order to avoid stopping the transmitter to send data while waiting for feedback information each time a packet is transmitted, a sliding window mechanism, well known in the art, is implemented.
In some implementations of this type, in case of transmission error, the transmitter retransmits all packets comprised in the sliding window even if some of them have been correctly received, well known as “Go-back-N” algorithms. As a result, a data packet overhead is generated by packet retransmissions. A solution to limit resource used by packet retransmissions consists in implementing a Selective Repeat scheme. In such a scheme, the feedback information message typically comprises the identifiers of incorrectly received packets, consequently only the incorrectly received packets are retransmitted by the transmitter. The Selective Repeat ARQ scheme can efficiently support data transmission with high throughputs and minimises the number of packet retransmission. However, an ARQ function on a receiver shall be able to periodically send feedback information messages to an ARQ function on a transmitter so that the sliding window can progress even when all packets are received correctly. Consequently, the amount of resource required for the feedback information messages transmission depends directly on the packet error rate since the amount of information sent in the feedback information messages, in this case, is function of the number of corrupted packets. At last, the amount of resource required for packets retransmission is proportional to the number of corrupted packets indicated in the feedback information messages. This type of scheme can be profitable to reduce the mean transmission delay as experienced by the upper layer. However, the resource consumed by the feedback information messages can be very important, mainly in case of transmission error bursts, above all when a Selective Repeat ARQ scheme is implemented. Consequently, another important aspect is to control the resource allocated for feedback transmission and the signalling overhead generated by the signalling protocol used to request a feedback resource allocation, as will be detailed in the following.
In a centralised resource allocation scheme, a specific device, called RRM unit, allocates the resource based on the received Resource Request messages sent by the different devices. A centralised Time Division Multiple Access (TDMA) MAC protocol based on a fixed MAC Frame Time Interval (FTI) is preferably adopted in such a scheme. When the transmitter ARQ function and the receiver ARQ function are not co-located within the RRM unit, an important signalling overhead may be generated by the EC entities. Indeed, the RRM unit first allocates a resource for the transmitter to allow the transmission of the Resource Request message from the transmitter to the RRM unit. Then, the RRM unit allocates a resource for data transmission from the transmitter to the receiver. Finally, the RRM unit allocates a resource for a feedback information transmission from the receiver to the transmitter.
To simplify this scheme and to limit the overhead generated by an ARQ scheme in order to save resource, the RRM unit can implicitly allocate resource in the backward direction, i.e. without exchanging any signalling messages. However, the RRM unit does not have knowledge of the state of the receiver ARQ function and consequently the resource allocation for feedback information messages performed by the RRM unit is not based on the transmission error detection. This solution can only be efficiently implemented when the RRM unit is co-located with the receiver ARQ function. If not, it may lead to either a lack of resource for feedback information messages when a burst of errors occurs, inducing an undetermined retransmission delay, or a waste of resource when all packets are correctly received. This signalling overhead can be accentuated with some operations performed by the PHY layer for synchronisation and channel estimation purposes, even if the size of transmitted data payload is small, which is generally the case for an ARQ signalling message. Moreover, the overall PHY layer overhead size depends, in the best case, on the number of transmitters in a given FTI. The PHY layer overhead can be significantly reduced if the number of transmitters in each FTI is reduced. FIG. 1 illustrates a feedback resource allocation scheme which is not implicitly performed. In case of transmission error detection, the transmitter ARQ function, respectively the receiver ARQ function sends a Resource Request message for data transmission 11, respectively a resource request message for signalling 12, to the RRM unit. The transmitter ARQ function retransmits data packets 14 to the receiver. Then, the receiver ARQ function sends a feedback information message to the transmitter via the resource 13 allocated by the RRM unit. FIG. 2 illustrates a usage of resource within the FTIs FTI#1, FTI #2 and FTI #3. In the FTI FTI#1, the transmitter ARQ function transmits a Resource Request message 21. Upon reception of this message 21, the RRM unit allocates in the next FTI which is the FTI FTI#2, the resource to the transmitter used to send a data packet 22. In the FTI FTI#2, the transmitter ARQ function sends another Resource Request message 23 to the RRM unit and then it is allocated a resource in the next FTI which is the FTI FTI#3. This resource is used by the transmitter to send a data packet 24. On the other hand, the receiver ARQ function sends a Resource Request message 26 to the RRM unit in the FTI FTI#2 in order to request a feedback resource. Consequently, the RRM unit allocates in FTI FTI#3 a resource to the receiver, used to send a feedback information message 27. The receiver ARQ function requests a feedback resource in the FTI FTI#3 via a Resource Request message 28 to be able to transmit the feedback information message in a next FTI. In such a feedback resource allocation scheme, the receiver ARQ function periodically requests some feedback resource via signalling messages.
Summarizing the preceding, an EC entity is very useful, mainly within networks using unreliable medium, such as wireless networks. But these types of networks manage a scarce resource and the known mechanisms required by an EC entity consume a lot of resource as it has been explained above. Actually, an EC entity requires feedback information messages, packets retransmission messages and consequently a feedback resource allocation scheme. It is to be noted here that a feedback resource allocation scheme consumes resource using specific resource allocation signalling messages when the allocation is not implicitly performed. Moreover, when a given EC scheme is designed to support high error rates, it generates resource waste in free-error transmission. As opposed to that, when a given EC scheme is designed to support to a low error rate, it is not adapted to high error rates, as it has been explained above. Stated otherwise, these types of EC schemes generate signalling overhead and/or packet retransmission overhead.